Recently, three-dimensional image display methods have been undergoing extensive study together with computer graphics. Since holography is a kind of photographic method, utilization thereof is very limited, although holography has excellent potential as a three-dimensional display method.
FIG. 9 schematically illustrates a principle of holography in the prior art. In FIG. 9, numeral 901 denotes a laser, 902 and 903 mirrors, 904 a beam splitter, 905, 906 and 907 lenses and 908 a photographic film. Operation of the holography apparatus configured above is now described.
A laser beam emitted from the laser 901 is divided into two directions by the beam splitter 904. The divided laser beams are spread through the lenses 905, 906 and 907 and are then overlapped with each other spatially. Interference fringes are formed in the direction of oblique lines in the space in which the laser beams are overlapped with each other (hatched portion in FIG. 9). A high-resolution photographic film 908 is set in this space so that both of the laser beams impinge on a recording surface of the film and the recording surface is exposed to the laser beams to record a pattern of the interference fringes on the recording surface. (An interval between the interference fringes is about 2 to 3 .mu.m and the photographic film on which the pattern has been recorded and developed is denominated the hologram. Further, the pattern is hereinafter referred to as hologram information.)
As shown in FIG. 10A, when the hologram 1001 is illuminated with one of the laser beams of FIG. 9, for example, parallel light (parallel reference light) passing through the lens 906, part of the laser beam is diffracted by the hologram and transmitted in the same direction as that of object light passing through the lens 907. Consequently, a virtual image A' is produced at a position of a point object A and the hologram is reproduced. Then, as shown in FIG. 10B, when the hologram is illuminated from the quite opposite direction to that of FIG. 10A by the laser beam with the same wave front (parallel beam in this case), the diffracted light is also transmitted in the opposite direction to that of FIG. 10A and is converged into a point A" (real image).
However, the above configuration has problems as follows:
(1) The resolution (about 0.1 line/.mu.m in the resolving power) of the sensor which can be utilized as an electrical detection device for the hologram information is very low as compared with the resolution (about 1 to 2 lines/.mu.m in resolving power) of the photographic material used as the hologram and accordingly it is difficult to detect the hologram information.
(2) An actual holographic television apparatus which can be operated in real time by using holography has not been achieved because of the drawback that there is no actual display device which can display the hologram information electrically.
In such circumstances, the present inventor has proposed a system utilizing three-dimensional hologram information, although once an available imaging element and image displaying element (refer to U.S. Pat. No. 4,654,685) require a three-dimensional structure and the technique of fabricating a three-dimensional integrated circuit is undeveloped at the present time, there is no present prospect for realization.